Next Article in Journal
Cerium-, Europium- and Erbium-Modified ZnO and ZrO2 for Photocatalytic Water Treatment Applications: A Review
Previous Article in Journal
Modified Catalysts and Their Fractal Properties
Previous Article in Special Issue
Modified Zeolite Catalyst for a NOx Selective Catalytic Reduction Process in Nitric Acid Plants
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 11
Issue 12
10.3390/catal11121519
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review

by
Hao Sun
and
Soo-Jin Park
*
Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(12), 1519; https://doi.org/10.3390/catal11121519
Submission received: 28 October 2021 / Revised: 9 December 2021 / Accepted: 13 December 2021 / Published: 14 December 2021
(This article belongs to the Special Issue Developments of Catalysts for the Selective Catalytic Reduction of NO with NH3)
Browse Figures
Versions Notes

Abstract

:
Recently, manganese oxides (MnOx)/cerium(IV) oxide (CeO2) composites have attracted widespread attention for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), which exhibit outstanding catalytic performance owing to unique features, such as a large oxygen storage capacity, excellent low-temperature activity, and strong mechanical strength. The intimate contact between the components can effectively accelerate the charge transfer to enhance the electron–hole separation efficiency. Nevertheless, MnOx/CeO2 still reveals some deficiencies in the practical application process because of poor thermal stability, and a low reduction efficiency. Constructing MnOx/CeO2 with other semiconductors is the most effective strategy to further improve catalytic performance. In this article, we discuss progress in the field of MnOx/CeO2-based ternary composites with an emphasis on the SCR of NOx by NH3. Recent progress in their fabrication and application, including suitable examples from the relevant literature, are analyzed and summarized. In addition, the interaction mechanisms between MnOx/CeO2 catalysts and NOx pollutants are comprehensively dissected. Finally, the review provides basic insights into prospects and challenges for the advancement of MnOx/CeO2-based ternary catalysts.

1. Introduction

Nitrogen oxides (NOx) are mainly generated by human activities, such as automobile exhaust, fossil fuels combustion, and chemical industry emissions, which may induce multiple environmental problems, including acid rain, ozone depletion, and greenhouse effects [1,2,3,4,5,6,7]. With the gradual progression of modern industrialization and urbanization, NOx emission has become a major environmental issue [8,9,10]. Therefore, NOx removal by applying an eco-friendly and sustainable technology is urgently required [11,12]. The specific catalytic reduction (SCR) of NOx with NH3 is a widely used approach for the degradation of pollutants, and plays an important role in energy conservation and emission reduction [13,14,15,16,17,18,19,20,21,22,23,24,25].
Thus far, various catalytic materials has been explored for the degradation of pollutants. Among them, MnOx/CeO2 composites have been extensively employed as highly efficient catalysts for the reduction of NOx with NH3 because of their unique characteristics, such as their simple synthesis, stable chemical structure, and excellent low temperature efficiency [25,26,27,28,29,30,31]. The introduction of manganese cations into a ceria lattice immensely enhances oxygen mobility in mixed oxides. In addition, MnOx/CeO2 composites with hollow structures possess high NOx storage capacity at low temperatures and excellent catalytic performance for the reduction of NOx owing to the large specific surface area, sufficient active sites, and their characteristic confined microenvironment [32,33,34,35,36,37,38,39,40,41,42,43,44]. However, the catalytic activity of MnOx/CeO2 composites is still deficient due to their low sulfur resistance and poor thermal stability [25,45,46]. As a result, the hybridization of MnOx/CeO2 with other semiconductors has been widely applied to improve performance. Various efforts to enhance the catalytic activity of these materials, such as the loading of noble metals, doping with extraneous elements, modifying the surface morphology, and fabricating heterojunction composites with other semiconductor materials, have been explored [47,48,49,50,51,52,53,54]. Among these, the fabrication of heterojunction composite systems can create more redox reaction sites and provides higher tenability and controllability, which is an effective strategy for improving the catalytic activity applied in environmental remediation [55,56,57,58]. Thus, combining MnOx/CeO2 with other semiconductors has been extensively studied. For instance, Zhang et al. [59] prepared MnOx-CeO2/TiO2 ternary catalysts that showed greatly enhanced catalytic activity for removing NOx with NH3 at low temperature compared to single semiconductor materials. In addition, Wang et al. [60] developed MnOx/CeO2/Al2O3 hybrid composite catalysts that achieved a NOx conversion greater than 90% due to their enhanced pore structure and a large specific surface area.
Presently, reviews of the preparation and application of MnOx/CeO2-based ternary catalysts are still rare. In this review, the research progress of MnOx/CeO2-based ternary composites and their use in the catalytic reduction of NOx with NH3 are summarized and proposed. Finally, we present some perspectives of the present circumstances and further prospects to further improve the understanding and extensive application of MnOx/CeO2-based ternary nanocomposites.

2. The Preparation and Catalytic Mechanism of MnOx/CeO2

2.1. The Preparation of MnOx/CeO2

MnOx/CeO2 can possess different structures and chemical performances incurred by various synthetic approaches and designs. Among them, the co-precipitation method is an important way to construct composites owing to the simple operation, manageable reaction conditions, and good products with uniform composition. In addition, the morphology and structure of the sample can be controlled by adjusting the reaction conditions, such as the solution pH value, temperature, etc. For instance, Ye et al. [61] successfully synthesized MnOx/CeO2 catalyst via a conventional co-precipitation method by blending manganese nitrate (Mn(NO3)2) with cerium nitrate (Ce(NO3)3·6H2O) at room temperature. In detail, a 12.5 wt.% ammonia solution was added dropwise to a solution containing 50 wt.% Mn(NO3)2 and Ce(NO3)3·6H2O at various molar ratios. The pH of the resulting solution was kept at 10.5 and stirred for 2 h. Finally, the as-obtained samples were collected by filtered washing several times with deionized water and ethyl alcohol, drying for 12 h at 105 °C, and the calcined for 6 h at 650 °C under an air atmosphere Qi et al. [27] developed a novel MnOx/CeO2 catalyst that achieved 95% NO conversion at 150 °C. Li et al. [62] reported MnOx/CeO2 prepared via a similar procedure, reaching 99% NOx conversion at 170 °C and 80% NO conversion rate in the temperature range of 120–275 °C; compared to the above-mentioned catalysts, it possess highest NOx conversion efficiency. Moreover, the main advantage of the hydrothermal method is that well-crystallized products can be easily obtained. Liu et al. [26] developed a novel MnOx/CeO2 catalyst with a shell-in-shell microspherical structure through a one-step hydrothermal method. In this process, 3.0 g of urea was completely dissolved in 100 mL deionized water. Subsequently, a certain amount of CeCl3·7H2O and Mn(NO3)2 were added under continuous stirring to form a clear solution. Afterward, the obtained mixture was transferred to a Teflon-sealed stainless-steel autoclave and heated for 180 °C for 20 h. The final product was filtered, washed with deionized water several times, and then dried at 80 °C for 12 h. Li et al. [63] evaluated a novel MnOx-CeO2 nanosphere catalyst with an assembled structure. It showed that the catalyst achieved a NO conversion about 100% in the entire 125–250 °C range. Moreover, other approaches, such as chemical deposition and oxidation-reduction reaction, have also been frequently applied to synthesize MnOx/CeO2-based nanocomposites [35,64].
As a result, it can be found that the preparation means and morphology of MnOx/CeO2 can affect the performance of catalysts. The morphology and structure of a sample can be controlled by adjusting the reaction conditions, such as solution pH value, temperature, etc. The MnOx/CeO2 catalyst prepared using the co-precipitation method exhibited a higher catalytic activity. Relatively, the main advantage of the hydrothermal method is that well-crystallized product can be easily obtained. The preparation of MnOx/CeO2 binary nanocomposites using different methods is summarized in Table 1.

2.2. The Catalytic Mechanism of MnOx/CeO2 for SCR with NH3

A catalyst is a material that can potentially be used for environment purification. Electrons and holes are shifted to the surface of the catalyst and produce active species for redox reactions to remove pollutants. Figure 1 describes the mechanism for the SCR process of MnOx-CeO2 catalyst. There are two reaction mechanisms for the NH3-SCR process over the MnOx-CeO2 catalyst, the Eley–Rideal (E–R) mechanism and Langmuir–Hinshelwood (L–H) mechanism [65,66]. These depend mainly on the adsorption state of the NH3 species react with the adsorbed or gaseous NOx. The NH3-SCR over the MnOx-CeO2 catalyst mainly followed the L–H mechanism rather than the E–R mechanism [67,68]. The Ce3+ and Mn3+ sites in the MnOx/CeO2 can improve the transfer efficiency of the activated oxygen molecules to generate surface-adsorbed oxygen species. Firstly, the NO molecules react with the Mn4+ to generate NO+. The obtained NO+ can further transformed into NO2 through a combination of surface-adsorbed oxygen species. In the next moment, the adsorbed NH3 species combine with surface activated oxygen to form NH2–. Finally, N2 and H2O are obtained through the reaction of NO2 and NH2− [45]. The SCR reaction followed the L–H mechanism, which exhibited excellent catalytic performance. The NH3 species reacting with the adsorbed or gaseous NO depends mainly on the adsorption state, and their adsorption on the catalyst surface is the critical step in SCRs with NH3 [69,70,71]. The reaction pathways can be described using (1)–(4):
NO + Mn4+  →  NO+ + Mn3+
NO+ + O−   →   NO2
NH3 + O− + e−  →  NH2− + OH−
NO2 + NH2−  →   N2 + H2O + O−
The catalytic activity of MnOx/CeO2 greatly hinges on its abundant active sites, specific surface area, and unique morphology [31,72,73,74]. In addition, MnOx/CeO2 with hollow structures improved catalytic activity, thereby making it effective at removing contaminants.

3. The Resistance Effect to H2O and SO2

Water vapor is able to destroy the acid sites of a catalyst to generate the poisoning effect on SCR, especially at low temperatures. When the temperature rises, the catalyst can be reactivated with a reduction in the inhibition effect. Even under dry conditions, the catalytic activity is still affected by the H2O produced by SCR reactions. Water molecules in the catalytic process are regarded as an important factor. For the poisoning of SO2, the effect of SO2 in low-temperature can results from the formation of ammonium sulfate species on catalyst surface, causing deactivation of the SCR. In flue gas containing both H2O and SO2, the resistance effects is more significant. Qi et al. [27] found that the conversion of NO changed from 98 to 95% in the presence of SO2 and H2O at low temperatures. Li et al. [35] proposed that the NOx conversion decreases from 100% to about 84% in 8 h and the catalytic activity restores up to 90% after adding SO2. Therefore, introducing water-resistant and sulfur-resistant materials may be an effective approach to solve the poisoning of H2O and SO2.

4. The Construction of MnOx/CeO2-Based Ternary Catalysts

The synthetic approaches for catalysts are closely related to catalytic performance. Different preparation methods can strongly impact on the structure, specific surface area, absorption, and porosity of catalysts. These synthesis methods mainly include hydrothermal, precipitation, sol–gel, and impregnation. For MnOx/CeO2-based ternary catalysts, Table 2 summarized the type of catalysts, preparation method, reaction conditions, and catalytic performance.

4.1. Hydrothermal Methods

Hydrothermal methods are the most simple and economical for large-scale synthesis and make it easy to obtain well-crystallized products by adjusting reaction conditions [85,86,87,88]. MnOx/CeO2-based catalysts obtain a good morphology and structure due to the high temperature and pressure in the hydrothermal system.
For instance, novel MnOx/CeO2/graphene ternary catalysts was prepared as follows [25]: 0.01 g graphene was first dispersed in 100 mL deionized water, and then certain amounts of Mn(NO3)2·3H2O and Ce(NO3)3·6H2O were added to the suspension. After that, ammonia at 20 wt.% was added to the solution under constant ultrasonic agitation. The mixture was then transferred into a Teflon-sealed autoclave and heated at 130 °C for 12 h. The obtained product was centrifugated and washed with deionized water several times, and then dried at 100 °C for 12 h. For comparison, other samples (different Mn/Ce molar ratios) were prepared under the same conditions. Sun et al. [34] fabricated MnOx/CeO2/MOF catalysts for catalytic reaction. Briefly, a certain amount of MnCl2·4H2O, Ce(NO3)3·6H2O, and 2,5-dihydroxyterephthalic acid were added to dimethyl furan-ethanol-water. Subsequently, the suspension was transferred into a Teflon-lined stainless-steel autoclave, and heated in an oven at 135 °C for 24 h. The product was washed with methanol, and dried in a 80 °C vacuum oven. Finally, samples was obtained through calcination at 550 °C in air. Zhu et al. [75] produced MnOx/CeO2/reduced graphene oxide that exhibited a high SCR efficiency via a one-step hydrothermal method. In the synthesis process, aqueous graphene oxide was dissolved in deionized water and ammonia solution was added dropwise to the solution to keep the pH value at 11.5. An aqueous solution of Mn(NO3)2 and Ce(NO3)3·6H2O was then added, followed by stirring at room temperature. After that, the mixture was transferred to a Teflon-lined autoclave and heated in an oven at 180 °C for 12 h. The final product was obtained and washed several times with deionized water. However, the hydrothermal method cannot control the number of nanocomposites exactly, making it prone to agglomeration

4.2. Precipitation Method

Precipitation is the formation of a solid from a homogeneous solution caused by a precipitating agent. This method is widely applied to prepare catalysts because of the controllable reaction conditions, effortless operation, and well-proportioned products [89,90], and is the most common approach for constructing MnOx/CeO2-based ternary composites.
For instance, a mixed solution containing carbamide solution, NH3 (25 wt.%), and hydrogen peroxide was continuously added into aqueous solutions of titanium(IV) sulfate (Ti(SO4)2), manganese acetate (C4H6MnO4·4H2O), and Ce, Fe, and Ni nitrates. Subsequently, the resulting precipitant was collected by centrifugation, followed by washing with ethyl alcohol and deionized water. Finally, the mixture was dried at 105 °C for 12 h, and then calcined at 450 °C in a muffle furnace. Furthermore, Chang et al. [76] reported SnO2/MnOx/CeO2 catalysts for SCR of NOx with ammonia. In their technique, (NH4)2CO3 was dispersed as a precipitator into a mixed solution of SnCl4, Mn(NO3)2, and Ce(NO3)3. The obtained paste was collected by ultrasound for 2 h, followed by subsequent filtration, washing with DI water, and calcination at 500 °C for 6 h. A catalytic material, WO3-ZrO2, was synthesized using the precursors ammonium metatungstate and zirconyl nitrate hydrate at the appropriate ratios [46], and the preparation procedure is shown in Figure 2.

4.3. Sol–Gel Methods

The sol–gel technique can be used to obtain highly pure materials and tune their composition due to its low cost, energy, and time consumption [91,92,93]. This approach is well-established for the preparation of novel metal oxide nanoparticles and mixed-oxide materials, and is beneficial for the stability of the products. Therefore, sol–gel methods have been widely applied for constructing MnOx/CeO2-based ternary nanocomposites.
A novel MnOx/CeO2/Al2O3 composite has been synthesized for soot oxidation in the presence of NOx through a facile sol–gel process involving Ce(NO3)3·6H2O and manganese acetate as precursors [77]. In brief, these were dissolved, and the suspension was then stirred sufficiently to form a porous gel that was dried at 110 °C overnight followed by calcination at 500 °C under static air in a muffle furnace. Zhang et al. [78] prepared MnOx/CeO2/SBA-15 (Santa Barbara Amorphous-15) catalysts with a 3D network structure using a sol–gel method for NOx-assisted soot combustion. Typically, certain amounts of Ce(NO3)3·6H2O and Mn(NO3)2 in aqueous solution were first added to anhydrous ethanol. After that, 1.00 g of SBA-15 was added dropwise to the mixed suspension, which was then dried at room temperature followed by calcination at 500 °C for 5 h in air.

4.4. Impregnation Methods

Impregnation methods are fast, low cost, and allow controllable configuration in advance by tweaking some parameters, such as concentration and the temperature of the precursor [94,95,96]. For instance, Wu et al. [97] prepared Ba/MnOx/CeO2 catalysts via impregnation to be employed for soot oxidation with heat transfer limitations. The composites were impregnated with an aqueous solution of barium acetate (Ba(Ac)2). The final products were obtained by heating at 110 °C for 12 h, followed by calcination at 550 °C in air. In addition, MnOx/CeO2/TiO2 mixed-oxide catalysts have been synthesized with differing CeO2/TiO2 ratios via impregnation for application in low-temperature catalytic ozonation of NOx into HNO3 [79]. A conventional MnOx/CeO2 supported with VOx (vanadium oxides) catalyst (denoted as VOx/MnOx/CeO2) was synthesized via a wet-impregnation method (Figure 3) [30]. In this typical synthesis, the mixture of Ce(NO3)3·6H2O and KIT-6 were calcinated at 500 °C for 4 h in air. After cooling to ambient temperature, the samples were centrifuged twice with NaOH to remove the KIT-6 template. Finally, 3-D mesoporous CeO2 supported with VOx and MnOx binary metal oxides was prepared.

5. The Application for SCR of NOx by NH3

In recent years, NOx, as an atmospheric pollutants from industrial discharge, has widely aroused public concern and resulted in a serious threat to human health and life [98,99,100,101,102,103,104]. Therefore, removing NOx from air is imperative. Considerable attention has been paid to catalytic technologies owing to their excellent properties, such as high efficiency and low cost. Therefore, some articles about producing MnOx/CeO2 composites for the SCR of NOx with NH3 are summarized here.

5.1. MnOx/CeO2 Binary Catalyst for SCR of NOx by NH3

Ye et al. [61] developed MnOx/CeO2 using a conventional co-precipitation method to remove NOx by SCR with NH3 to leave N2. The catalytic performance of MnOx/CeO2 composites decrease significantly in the presence of toluene, which is attributed to the competitive adsorption of toluene and the deposition of by-products on the catalyst surface. Meanwhile, toluene adsorption enhanced the formation of oxygen vacancies and increased the unfavored oxidation reactions of NH3, leading to a decrease in N2 selectivity. Novel MnOx/CeO2 hollow nanotubes were applied to the low-temperature SCR of NOx with NH3 [35]. It was demonstrated that MnOx/CeO2 nanocomposites exhibited best conversion efficiency of NOx at 100 °C, and had superior resistance to H2O and SO2. According to the analysis results, the excellent NOx catalytic property of the MnOx/CeO2 catalyst could be ascribed to the uniform distribution and abundant content of active species, especially the hollow porous architectures that provided higher specific surface area.
Qi et al. [27] employed a co-precipitation method for the preparation of MnOx/CeO2 mixed oxides. X-ray diffraction (XRD), surface area measurements, and Fourier-transform infrared (FTIR) spectroscopy were employed to characterize MnOx/CeO2 catalyst. It was found that Mn-Ce mixed-oxide catalyst yielded 95% NO removal efficiency at 150 °C at a space velocity of 42,000 h−1. With increasing manganese weight ratios, NO conversion efficiency gradually increased, but decreased at higher manganese contents. The MnOx/CeO2 heterojunction represented an improved catalytic activity on account of the adsorption of NH3 on the Lewis acid sites of the catalyst, followed by reaction with nitrite species to produce N2.
More recently, Liu et al. [26] prepared novel MnOx/CeO2 shell-in-shell microspheres that showed better catalytic activity for SCR of NO compared with the CeMnOx catalyst without the core–shell structure. MnOx/CeO2 catalysts present great potential in terms of catalytic reduction of NOx due to the oxygen storage and redox property. The TEM and HRTEM images showed that a small hollow sphere was located in the inside of a big hollow sphere to construct a double-shelled hollow sphere (Figure 4). On the basis of the data from XPS measurements, it was observed that MnOx/CeO2 is composed of MnOx and CeO2.
Li et al. [63] reported that a novel MnOx/CeO2 nanosphere catalyst with assembled structure from tiny particle exhibited excellent catalytic performance for the reduction of NOx. Compared to pure MnOx and CeO2, MnOx/CeO2 nanosphere catalyst show a superior DeNOx performance because of the higher specific surface area, better redox behavior, and larger concentration of surface active oxygen species. The stability of MnOx/CeO2 nanosphere NH3-SCR catalyst achieved the best effect at 150 °C for long time. In addition, the pore diameter and BET surface area of the composite were higher than those of the single MnOx and CeO2 samples. The results of the study suggested that control of defined structural morphology can improve NH3-SCR performance.
Li et al. [62] successfully synthesized a mesoporous MnOx/CeO2 composites through a traditional co-precipitation route. The prepared catalyst exhibited enhanced performance for NOx conversion, which can be attributed to its smaller pores, amorphous structure, and moderate amount of surface Mn3+/oxygen species. Moreover, the porous structure of composites provided a larger surface area and made the adsorption and diffusion of reactant molecules easy to conduct on the surface of catalysts. The MnOx/CeO2 nanocomposite exhibited higher catalytic activity than pure MnOx and CeO2 for the SCR of NO with NH3.
Huang et al. [64] reported a CeO2/MnOx catalyst with a core-shell structure was prepared via a facile a chemical deposition method. The TEM images of the CeO2/MnOx heterostructure identified a distinct core–shell structure and a uniform size of the CeO2/MnOx and MnOx nanoparticles. The improvement in SCR performance is attributed to the uniform core–shell structure, the high crystalline α-MnO2, as well as the high concentration of Mn4+ and Ce3+ in the CeO2/MnOx solid structure.

5.2. MnOx/CeO2 Based Ternary Catalysts for SCR of NOx

The catalytic technique has attracted a great deal of attention and is widely investigated to apply SCR of NO using NH3. Especially, MnOx/CeO2-based ternary catalyst can significantly promote catalytic activity for environmental pollution, including the reduction of NOx [20,105]. Therefore, the application and properties of MnOx/CeO2-based ternary composites are summarized and discussed.
Sheng et al. [25] developed MnOx/CeO2/graphene catalysts prepared via a hydrothermal method using different molar ratios of Mn/Ce active components. The results in the morphology and microstructure indicated that manganese and cerium oxides were uniformly dispersed on the surface of graphene. The MnOx/CeO2 (8:1)/GR displayed the best catalytic activity, and a high N2 selectivity, which are attributed to the high content of chemisorbed oxygen on the surface, abundant active sites, and the synergistic effect between MnOx/CeO2 and the graphene support. Hence, the excellent catalytic performance of MnOx/CeO2/GR composites at low temperature were widely applied in controlling NOx emissions from flue gas.
Liu et al. [80] successfully developed a novel MnOx/CeO2 supported on Cu-SSZ-13 catalyst through an impregnation method. The Mn-Ce/Cu-SSZ-13 possessed higher catalytic activities than Cu-SSZ-13 and Mn-Ce for NOx conversions because the bridging nitrates were adsorbed onto the surface and then converted to monodentate nitrates. Therefore, ternary composites can obtain more active species for the SCR reaction at low temperatures.
Recently, Ma et al. [81] reported the synthesis of MnOx/CeO2@TiO2 core–shell composites. TEM and high-resolution TEM images reveal that TiO2 was distributed on the surface of MnOx/CeO2 nanorods to form the core–shell structure. MnOx/CeO2@TiO2 showed excellent catalytic performance due to abundant active sites, strong redox capability, along with a large specific surface area and pore volume. Meanwhile, the TiO2 shell of MnOx/CeO2@TiO2 further promotes catalytic activity and physicochemical properties.
A synthesized MnOx/CeO2/Al2O3 (aluminum oxide) composite catalyst achieved 90% NOx conversion from 150–300 °C [60]; its BET surface area and pore volume were approximately 207.5 m2/g and 0.23 cm3/g, respectively, while the MnOx/CeO2/Al2O3 ternary heterojunctions remarkably improved the catalytic performance and provided good stability through the large intimate interfacial contacts among the MnOx, CeO2, and Al2O3 constituents.
Zhu et al. [75] reported the fabrication of 3D MnOx/CeO2 nanoparticles/reduced graphene aerogel (RGA) via a facile one-step hydrothermal method. The MnOx/CeO2 nanoparticles were uniformly distributed on graphene nanosheets to form self-assembling 3D interconnected networks. The composites showed significantly enhanced catalytic performance compared to MnOx/CeO2 nanoparticles (99% NOx conversion at 220 °C). Therefore, MnOx/CeO2-based ternary composite materials have great prospects for the SCR of NOx at low temperatures. A schematic of the catalytic mechanism of MnOx/CeO2/RGA is shown in Figure 5.
Zhang et al. [82] successfully developed a MnOx/CeO2/TiO2 catalysts for SCR of NO with NH3 at low temperatures. The results of BET show that the Mn-Ce/TiO2 ternary catalyst has a large specific surface area and pore volume, which provided abundant active sites for improved catalytic activity. The XPS spectra of the prepared MnOx-CeO2/TiO2 was used to analyze the surface chemical and valence states, which demonstrated the formation of heterojunction. In addition, it found that the low-temperature SCR performance was prominently enhanced. Meanwhile, NO conversion efficiency achieved the best effect in the temperature range of 150–300 °C. Figure 6 depicts the K+ poisoning process of the catalyst.
MnOx/CeO2/ZrO2 monolith catalysts with a WO3 content of 10 wt.% were fabricated using a co-precipitation method, which had the best catalytic activity and the widest reaction window [46]. In addition, it possessed a better thermal stability and SO2 tolerance than pure MnOx/CeO2 materials. NOx conversion was more than 80% in the low temperature range at the space velocity of 10,000 h−1.
The construction of novel MnOx/CeO2/VOx catalysts was applied for selective catalytic reduction of NOx by NH3 [83]. The NH3-SCR performance of the catalytic experiment indicated that improved catalytic activity was achieved through the formation of MnOx/CeO2/VOx heterojunctions. The possible mechanisms of the VOx-MnOx/CeO2 catalyst are shown in Figure 7.
In research conducted by Liu et al. [76], MnOx/CeO2/SnO2 catalysts were prepared via a co-precipitation method for NH3-SCR reaction. The composite catalyst (10 wt.% of Sn on MnOx/CeO2) displayed optimal catalytic activity in a temperature range of 80–230 °C. The remarkable catalytic activity was likely attributed to the enhanced Lewis acid sites created by surface sulfation during the SO2-containing SCR reaction.
Wu et al. [30] employed an efficient wet impregnation method for the fabrication of heterojunction VOx-MnOx/CeO2 nanocomposites. TEM and HRTEM analyses were employed to investigate the microstructure and morphology of VOx-MnOx/CeO2 (Figure 8). The ternary nanocomposites exhibited improved conversion efficiency for NO because gaseous NH3 and NO are favorable to be absorbed on the surface of the VOx-MnOx/CeO2 catalyst. The results of the NH3-SCR performance indicated that the NO conversion reached 95% in a wide temperature range of 220–330 °C. Figure 9 shows the possible mechanisms of the VOx-MnOx/CeO2 catalyst.
Wang et al. [60]. evaluated a novel MnOx/CeO2/Al2O3 ternary catalysts with varying Mn contents through a self-propagating high-temperature synthesis. The characterization results showed that the increasing of the surface atomic concentration improved the activity in the SCR reaction. Moreover, MnOx/CeO2/Al2O3 possess an extensive pore structure and a large BET surface area to further promote the catalytic performances.
Lee et al. [84] successfully developed MnOx/CeO2/TiO2 composites for low-temperature SCR of NO with NH3. The Mn/Ce/TiO2 (20 wt.% Mn/4 wt.% Ce) catalyst exhibited the highest catalytic activity in the low-temperature range of 120–160 °C, which was mainly attributed to widespread Mn4+ dispersion on the surface.
These illustrations of MnOx/CeO2-based ternary catalysts for SCR of NOx provided basic perspectives and instructions for their application to air purification and environmental conservation.

6. Conclusions and Perspectives

In summary, the current developments in the preparation and application of MnOx/CeO2-based ternary catalysts for selective catalytic reduction of NOx by NH3 are summarized. Various synthetic routes, such as hydrothermal, precipitation, sol–gel, and impregnation methods for fabricating MnOx/CeO2-based ternary catalysts have been explored. Their large specific surface areas, abundant active sites, and excellent catalytic performances make them very effective materials for SCR of NOx with NH3. Nevertheless, despite significant improvements in MnOx/CeO2-based ternary catalysts, there are many challenges to overcome to further enhance their material properties and catalytic reaction mechanisms. Therefore, we offer some insights to consider for future research:
Further analyses are required to determine with the catalytic activity with various morphologies (nanofibers, nanotubes, nanorods) of MnOx/CeO2.
The reaction mechanisms of MnOx/CeO2 catalysts. The effects of catalyst properties (e.g., pore size, porosity, and surface structure) on catalytic activity should be further explored.
Application research should be expanded. Current applications of MnOx/CeO2-based hybrids are mainly targeted at nitrogen fixation and degradation of formaldehyde. MnOx/CeO2 catalysts can be explored for applications in the degradation of more pollutants, such as in Cr(VI) reduction and CO2 reduction.
Current research has mainly been done under laboratory conditions. For practical engineering applications, the design of applicable reaction systems for MnOx/CeO2 catalysts is required.
The transfer mechanism of charge carriers needs to be better understood as understanding of the mechanism is helpful to seek more efficient materials that can combine with MnOx/CeO2 to achieve better catalytic activity.

Author Contributions

H.S.: Designing experiment, investigation, optimizing methodology, experimental and data analysis, writing-original draft, review & editing. S.-J.P.: conceptualization, supervision, finalizing, writing-review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Korea Electric Power Corporation (Grant number: R21XO01-5). This work also supported by the Korea Industrial Complex Corporation through the Project to Program of Industrial Complex Clusters (IRIC2101, Industry-University-Research joint R&D support) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Kumar, B.R.; Saravanan, S.; Rana, D.; Anish, V.; Nagendran, A. Effect of a sustainable biofuel–n-octanol–on the combustion, performance and emissions of a DI diesel engine under naturally aspirated and exhaust gas recirculation (EGR) modes. Energy Convers. Manag. 2016, 118, 275–286. [Google Scholar] [CrossRef]
  2. Richter, A.; Burrows, J.P.; Nüß, H.; Granier, C.; Niemeier, U. Increase in tropospheric nitrogen dioxide over China observed from space. Nature 2005, 437, 129–132. [Google Scholar] [CrossRef] [PubMed]
  3. Reşitoğlu, İ.A.; Altinişik, K.; Keskin, A. The pollutant emissions from diesel-engine vehicles and exhaust aftertreatment systems. Clean Technol. Environ. Policy 2015, 17, 15–27. [Google Scholar] [CrossRef] [Green Version]
  4. Song, W.; Liu, X.Y.; Hu, C.C.; Chen, G.Y.; Liu, X.J. Very low mutation burden is a feature of inflamed recurrent glioblastomas responsive to cancer immunotherapy. Nat. Commun. 2021, 12, 1–7. [Google Scholar]
  5. Liu, M.; Huang, X.; Song, Y.; Tang, J.; Cao, J.; Zhang, X.; Zhang, Q.; Wang, S.; Xu, T.; Kang, L.; et al. Ammonia emission control in China would mitigate haze pollution and nitrogen deposition, but worsen acid rain. Proc. Natl. Acad. Sci. USA 2019, 116, 7760–7765. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  6. Sun, Y.; Zwolińska, E.; Chmielewski, A.G. Abatement technologies for high concentrations of NOx and SO2 removal from exhaust gases: A review. Crit. Rev. Environ. Sci. Technol. 2016, 46, 119–142. [Google Scholar] [CrossRef]
  7. Derwent, R.G.; Collins, W.J.; Johnson, C.E.; Stevenson, D.S. Transient behaviour of tropospheric ozone precursors in a global 3-D CTM and their indirect greenhouse effects. Clim. Change 2001, 49, 463–487. [Google Scholar] [CrossRef]
  8. Xu, S.C.; Miao, Y.M.; Gao, C.; Long, R.Y.; Chen, H.; Zhao, B.; Wang, S.X. Regional differences in impacts of economic growth and urbanization on air pollutants in China based on provincial panel estimation. J. Clean. Prod. 2019, 208, 340–352. [Google Scholar] [CrossRef]
  9. Liu, C.; Yang, M. An empirical analysis of dynamic changes in ecological sustainability and its relationship with urbanization in a coastal city: The case of Xiamen in China. J. Clean. Prod. 2020, 256, 120482. [Google Scholar] [CrossRef]
  10. Carslaw, D.C. Evidence of an increasing NO2/NOx emissions ratio from road traffic emissions. Atmospheric Environ. 2005, 39, 4793–4802. [Google Scholar] [CrossRef]
  11. Seddiek, I.S.; Elgohary, M.M. Eco-friendly selection of ship emissions reduction strategies with emphasis on SOx and NOx emissions. Int. J. Nav. Arch. Ocean 2014, 6, 737–748. [Google Scholar] [CrossRef] [Green Version]
  12. Chung, S.J.; Pillai, K.C.; Moon, I.S. A sustainable environmentally friendly NOx removal process using Ag (II)/Ag (I)-mediated electrochemical oxidation. Sep. Purif. Technol. 2009, 65, 156–163. [Google Scholar] [CrossRef]
  13. Yao, X.; Zhao, R.; Chen, L.; Du, J.; Tao, C.; Yang, F.; Dong, L. Selective catalytic reduction of NOx by NH3 over CeO2 supported on TiO2: Comparison of anatase, brookite, and rutile. Appl. Catal. B 2017, 208, 82–93. [Google Scholar] [CrossRef]
  14. Xie, L.; Liu, F.; Shi, X.; Xiao, F.S.; He, H. Effects of post-treatment method and Na co-cation on the hydrothermal stability of Cu–SSZ-13 catalyst for the selective catalytic reduction of NOx with NH3. Appl. Catal. B 2015, 179, 206–212. [Google Scholar] [CrossRef]
  15. Li, X.; Li, K.; Peng, Y.; Li, X.; Zhang, Y.; Wang, D.; Chen, J.; Li, J. Interaction of phosphorus with a FeTiOx catalyst for selective catalytic reduction of NOx with NH3: Influence on surface acidity and SCR mechanism. Chem. Eng. J. 2018, 347, 173–183. [Google Scholar] [CrossRef]
  16. Zong, L.; Zhang, G.; Zhao, J.; Dong, F.; Zhang, J.; Tang, Z. Morphology-controlled synthesis of 3D flower-like TiO2 and the superior performance for selective catalytic reduction of NOx with NH3. Chem. Eng. J. 2018, 343, 500–511. [Google Scholar] [CrossRef]
  17. Wang, Q.; Xu, H.; Huang, W.; Pan, Z.; Zhou, H. Metal organic frameworks-assisted fabrication of CuO/Cu2O for enhanced selective catalytic reduction of NOx by NH3 at low temperatures. J. Hazard. Mater. 2019, 364, 499–508. [Google Scholar] [CrossRef]
  18. Gómez-García, M.A.; Pitchon, V.; Kiennemann, A. Pollution by nitrogen oxides: An approach to NOx abatement by using sorbing catalytic materials. Environ. Int. 2005, 31, 445–467. [Google Scholar] [CrossRef]
  19. Han, K.; Luo, J.; Feng, Y.; Lai, Q.; Bai, Y.; Tang, W.; Wang, Z.L. Wind-driven radial-engine-shaped triboelectric nanogenerators for self-powered absorption and degradation of NOX. ACS Nano 2020, 14, 2751–2759. [Google Scholar] [CrossRef] [PubMed]
  20. Yan, R.; Lin, S.; Li, Y.; Liu, W.; Mi, Y.; Tang, C.; Wang, L.; Wu, P.; Peng, H. Novel shielding and synergy effects of Mn-Ce oxides confined in mesoporous zeolite for low temperature selective catalytic reduction of NOx with enhanced SO2/H2O tolerance. J. Hazard. Mater. 2020, 396, 122592. [Google Scholar] [CrossRef]
  21. Wang, J.; Zhao, H.; Haller, G.; Li, Y. Recent advances in the selective catalytic reduction of NOx with NH3 on Cu-Chabazite catalysts. Appl. Catal. B 2017, 202, 346–354. [Google Scholar] [CrossRef]
  22. Bai, Y.; Dong, J.; Hou, Y.; Guo, Y.; Liu, Y.; Li, Y.; Han, X.; Huang, Z. Co3O4@PC derived from ZIF-67 as an efficient catalyst for the selective catalytic reduction of NOx with NH3 at low temperature. Chem. Eng. J. 2019, 361, 703–712. [Google Scholar] [CrossRef]
  23. Kwon, D.W.; Park, K.H.; Ha, H.P.; Hong, S.C. The role of molybdenum on the enhanced performance and SO2 resistance of V/Mo-Ti catalysts for NH3-SCR. Appl. Surf. Sci. 2019, 481, 1167–1177. [Google Scholar] [CrossRef]
  24. Saramok, M.; Szymaszek, A.; Inger, M.; Jurak, K.A.; Samojeden, B.; Motak, M. Modified Zeolite Catalyst for a NOx Selective Catalytic Reduction Process in Nitric Acid Plants. Catalysts 2021, 11, 450. [Google Scholar] [CrossRef]
  25. You, X.; Sheng, Z.; Yu, D.; Yang, L.; Xiao, X.; Wang, S. Influence of Mn/Ce ratio on the physicochemical properties and catalytic performance of graphene supported MnOx-CeO2 oxides for NH3-SCR at low temperature. Appl. Surf. Sci. 2017, 423, 845–854. [Google Scholar] [CrossRef]
  26. Liu, C.; Gao, G.; Shi, J.W.; He, C.; Li, G.; Bai, N.; Niu, C. MnOx-CeO2 shell-in-shell microspheres for NH3-SCR de-NOx at low temperature. Catal. Commun. 2016, 86, 36–40. [Google Scholar] [CrossRef]
  27. Qi, G.; Yang, R.T.; Chang, R. MnOx-CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal. B 2004, 51, 93–106. [Google Scholar] [CrossRef]
  28. Qi, G.; Yang, R.T. Performance and kinetics study for low-temperature SCR of NO with NH3 over MnOx–CeO2 catalyst. J Catal. 2003, 217, 434–441. [Google Scholar] [CrossRef]
  29. Ye, L.; Lu, P.; Peng, Y.; Li, J.; Huang, H. Impact of NOx and NH3 addition on toluene oxidation over MnOx-CeO2 catalyst. J. Hazard. Mater. 2021, 416, 125939. [Google Scholar] [CrossRef]
  30. Wu, X.; Chen, Z.; Yu, X.; Huang, Z.; Shen, H.; Jing, G. Mechanism by which three-dimensional ordered mesoporous CeO2 promotes the activity of VOx-MnOx/CeO2 catalysts for NOx abatement: Atomic-scale insight into the catalytic cycle. Chem. Eng. J. 2020, 399, 125629. [Google Scholar] [CrossRef]
  31. Andreoli, S.; Deorsola, F.A.; Pirone, R. MnOx-CeO2 catalysts synthesized by solution combustion synthesis for the low-temperature NH3-SCR. Catal. Today 2015, 253, 199–206. [Google Scholar] [CrossRef]
  32. Zhang, H.; Yuan, S.; Wang, J.L.; Gong, M.; Chen, Y. Effects of contact model and NOx on soot oxidation activity over Pt/MnOx-CeO2 and the reaction mechanisms. Chem. Eng. J. 2017, 327, 1066–1076. [Google Scholar] [CrossRef]
  33. Delimaris, D.; Ioannides, T. VOC oxidation over MnOx–CeO2 catalysts prepared by a combustion method. Appl. Catal. B 2008, 84, 303–312. [Google Scholar] [CrossRef]
  34. Sun, H.; Yu, X.; Ma, X.; Yang, X.; Lin, M.; Ge, M. MnOx-CeO2 catalyst derived from metal-organic frameworks for toluene oxidation. Catal. Today 2019, 35, 580–586. [Google Scholar] [CrossRef]
  35. Li, C.; Tang, X.; Yi, H.; Wang, L.; Cui, X.; Chu, C.; Li, J.; Zhang, R.; Yu, Q. Rational design of template-free MnOx-CeO2 hollow nanotube as de-NOx catalyst at low temperature. Appl. Surf. Sci. 2018, 428, 924–932. [Google Scholar] [CrossRef]
  36. Shen, B.; Wang, F.; Liu, T. Homogeneous MnOx–CeO2 pellets prepared by a one-step hydrolysis process for low-temperature NH3-SCR. Powder Technol. 2014, 253, 152–157. [Google Scholar] [CrossRef]
  37. Tikhomirov, K.; Kröcher, O.; Elsener, M.; Wokaun, A. MnOx-CeO2 mixed oxides for the low-temperature oxidation of diesel soot. Appl. Catal. B 2006, 64, 72–78. [Google Scholar] [CrossRef]
  38. Cui, M.; Li, Y.; Wang, X.; Wang, J.; Shen, M. Effect of preparation method on MnOx-CeO2 catalysts for NO oxidation. J. Rare Earths. 2013, 31, 572–576. [Google Scholar] [CrossRef]
  39. Li, H.; Wu, C.Y.; Li, Y.; Li, L.; Zhao, Y.; Zhang, J. Role of flue gas components in mercury oxidation over TiO2 supported MnOx-CeO2 mixed-oxide at low temperature. J. Hazard. Mater. 2012, 243, 117–123. [Google Scholar] [CrossRef]
  40. Wang, X.; Kang, Q.; Li, D. Catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts. Appl. Catal. B 2009, 86, 166–175. [Google Scholar]
  41. Kong, Z.; Li, Y.; Wang, Y.; Zhang, Y.; Shen, K.; Chu, X.; Wang, H.; Wang, J.; Zhan, L. Monodispersed MnOx-CeO2 solid solution as superior electrocatalyst for Li2S precipitation and conversion. Chem. Eng. J. 2020, 392, 123697. [Google Scholar] [CrossRef]
  42. Tang, X.; Chen, J.; Huang, X.; Xu, Y.; Shen, W. Pt/MnOx–CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature. Appl. Catal. B 2008, 81, 115–121. [Google Scholar] [CrossRef]
  43. Xie, Y.; Li, C.; Zhao, L.; Zhang, J.; Zeng, G.; Zhang, X.; Zhang, W.; Tao, S. Experimental study on Hg0 removal from flue gas over columnar MnOx-CeO2/activated coke. Appl. Surf. Sci. 2015, 333, 59–67. [Google Scholar] [CrossRef]
  44. Meng, Q.; Cui, J.; Wang, K.; Tang, Y.; Zhao, K.; Huang, L. Facile preparation of hollow MnOx-CeO2 composites with low Ce content and their catalytic performance in NO oxidation. Mol. Catal. 2020, 493, 111107. [Google Scholar] [CrossRef]
  45. Yao, X.; Ma, K.; Zou, W.; He, S.; An, J.; Yang, F.; Dong, L. Influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for NH3-SCR at low temperature. Chinese J. Catal. 2017, 38, 146–159. [Google Scholar] [CrossRef]
  46. Xu, H.; Zhang, Q.; Qiu, C.; Lin, T.; Gong, M.; Chen, Y. Tungsten modified MnOx–CeO2/ZrO2 monolith catalysts for selective catalytic reduction of NOx with ammonia. Chem. Eng. Sci. 2012, 76, 120–128. [Google Scholar] [CrossRef]
  47. Sun, H.; Park, S.J. Phosphorus-doped g-C3N4/SnS nanocomposite for efficient photocatalytic reduction of aqueous Cr (VI) under visible light. Appl. Surf. Sci. 2020, 531, 147325. [Google Scholar] [CrossRef]
  48. Zhang, P.; Lou, X.W. Design of Heterostructured Hollow Photocatalysts for Solar-to-Chemical Energy Conversion. Adv. Mater. 2019, 31, 1900281. [Google Scholar] [CrossRef] [PubMed]
  49. Wang, Z.; Lv, J.; Zhang, J.; Dai, K.; Liang, C. Facile synthesis of Z-scheme BiVO4/porous graphite carbon nitride heterojunction for enhanced visible-light-driven photocatalyst. Appl. Surf. Sci. 2018, 430, 595–602. [Google Scholar] [CrossRef]
  50. Yang, X.; Tian, L.; Zhao, X.; Tang, H.; Liu, Q.; Li, G.X. Interfacial optimization of g-C3N4-based Z-scheme heterojunction toward synergistic enhancement of solar-driven photocatalytic oxygen evolution. Appl. Catal. B 2019, 244, 240–249. [Google Scholar] [CrossRef]
  51. Marschall, R. Semiconductor composites: Strategies for enhancing charge carrier separation to improve photocatalytic activity. Adv. Funct. Mater. 2014, 24, 2421–2440. [Google Scholar] [CrossRef]
  52. Liang, Y.; Guo, N.; Li, L.; Li, R.; Ji, G.; Gan, S. Fabrication of porous 3D flower-like Ag/ZnO heterostructure composites with enhanced photocatalytic performance. Appl. Surf. Sci. 2015, 332, 32–39. [Google Scholar] [CrossRef]
  53. Di, L.; Yang, H.; Xian, T.; Liu, X.; Chen, X. Photocatalytic and photo-Fenton catalytic degradation activities of Z-scheme Ag2S/BiFeO3 heterojunction composites under visible-light irradiation. Nanomaterials 2019, 9, 399. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  54. El-Bery, H.M.; Matsushita, Y.; Abdel-moneim, A. Fabrication of efficient TiO2-RGO heterojunction composites for hydrogen generation via water-splitting: Comparison between RGO, Au and Pt reduction sites. Appl. Surf. Sci. 2017, 423, 185–196. [Google Scholar] [CrossRef]
  55. Song, H.; Wu, R.; Yang, J.; Dong, J.; Ji, G. Fabrication of CeO2 nanoparticles decorated three-dimensional flower-like BiOI composites to build pn heterojunction with highly enhanced visible-light photocatalytic performance. J. Colloid Interface Sci. 2018, 512, 325–334. [Google Scholar] [CrossRef] [PubMed]
  56. Wang, Q.; Wang, W.; Zhong, L.; Liu, D.; Cao, X.; Cui, F. Oxygen vacancy-rich 2D/2D BiOCl-g-C3N4 ultrathin heterostructure nanosheets for enhanced visible-light-driven photocatalytic activity in environmental remediation. Appl. Catal. B 2018, 220, 290–302. [Google Scholar] [CrossRef]
  57. Li, L.; Yu, X.; Xu, L.; Zhao, Y. Fabrication of a novel type visible-light-driven heterojunction photocatalyst: Metal-porphyrinic metal organic framework coupled with PW12/TiO2. Chem. Eng. J. 2020, 386, 123955. [Google Scholar] [CrossRef]
  58. Dutta, V.; Sharma, S.; Raizada, P.; Thakur, V.K.; Khan, A.A.P.; Saini, V.; Asiri, A.M.; Singh, P.V. An Overview on WO3 based photocatalyst for environmental remediation, J. Environ. Chem. Eng. 2021, 9, 105018. [Google Scholar]
  59. Li, H.; Wu, C.Y.; Li, Y.; Zhang, J. Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures. Appl. Catal. B 2012, 111, 381–388. [Google Scholar] [CrossRef]
  60. Wang, C.; Yu, F.; Zhu, M.; Tang, C.; Zhang, K.; Zhao, D.; Dong, L.; Dai, B. Bin. Dai, Highly selective catalytic reduction of NOx by MnOx–CeO2–Al2O3 catalysts prepared by self-propagating high-temperature synthesis. J. Environ. Sci. 2019, 75, 124–135. [Google Scholar] [CrossRef] [PubMed]
  61. Ye, L.; Lu, P.; Chen, X.; Fang, P.; Peng, Y.; Li, J.; Huang, H. The deactivation mechanism of toluene on MnOx-CeO2 SCR catalyst. Appl. Catal. B 2020, 277, 119257. [Google Scholar] [CrossRef]
  62. Li, W.; Liu, H.; Chen, Y. Mesoporous MnOx–CeO2 composites for NH3-SCR: The effect of preparation methods and a third dopant. RSC Adv. 2019, 9, 11912–11921. [Google Scholar]
  63. Li, L.; Sun, B.; Sun, J.; Yu, S.; Ge, C.; Tang, C.; Dong, L. Novel MnOx-CeO2 nanosphere catalyst for low-temperature NH3-SCR. Catal Commun. 2017, 100, 98–102. [Google Scholar] [CrossRef]
  64. Li, S.; Huang, B.; Yu, C. A CeO2-MnOx core-shell catalyst for low-temperature NH3-SCR of NO. Catal. Commun. 2017, 98, 47–51. [Google Scholar] [CrossRef]
  65. Yao, X.; Chen, L.; Cao, J.; Chen, Y.; Tian, M.; Yang, F.; Sun, J.; Tang, C.; Dong, L. Enhancing the deNOx performance of MnOx/CeO2-ZrO2 nanorod catalyst for low-temperature NH3-SCR by TiO2 modification. Chem. Eng. J. 2019, 369, 46–56. [Google Scholar] [CrossRef]
  66. Anthonysamy, S.B.I.; Afandi, S.B.; Khavarian, M.; Mohamed, A.R.B. A review of carbon-based and non-carbon-based catalyst supports for the selective catalytic reduction of nitric oxide. Beilstein J. Nanotechnol. 2018, 9, 740–761. [Google Scholar] [CrossRef]
  67. Ma, L.; Seo, C.Y.; Nahata, M.; Chen, X.; Li, J.; Schwank, J.W. Shape dependence and sulfate promotion of CeO2 for selective catalytic reduction of NOx with NH3. Appl. Catal. B 2018, 232, 246–259. [Google Scholar] [CrossRef]
  68. Liu, C.; Shi, J.W.; Gao, C.; Niu, C. Manganese oxide-based catalysts for low-temperature selective catalytic reduction of NOx with NH3: A review. Appl. Catal. A Gen. 2016, 522, 54–69. [Google Scholar] [CrossRef]
  69. Ma, Z.; Weng, D.; Wu, X.; Si, Z. Effects of WOx modification on the activity, adsorption and redox properties of CeO2 catalyst for NOx reduction with ammonia. J. Environ. Sci. 2012, 24, 1305–1316. [Google Scholar] [CrossRef]
  70. Chatterjee, D.; Burkhardt, T.; Weibel, M.; Tronconi, E.; Nova, I.; Ciardelli, C. Numerical Simulation of NO/NO2/NH3 Reactions on SCR-Catalytic Converters: Model Development and Applications; SAE Technical Paper No. 2006-01-0468; SAE International: Warrendale, PA, USA, 2006. [Google Scholar]
  71. Xu, Y.; Wu, X.; Lin, Q.; Hu, J.; Ran, R.; Weng, D. SO2 promoted V2O5-MoO3/TiO2 catalyst for NH3-SCR of NOx at low temperatures. Appl. Catal. A Gen. 2019, 570, 42–50. [Google Scholar] [CrossRef]
  72. Jiang, Y.; Gao, J.; Zhang, Q.; Liu, Z.; Fu, M.; Wu, J.; Hu, Y.; Ye, D. Enhanced oxygen vacancies to improve ethyl acetate oxidation over MnOx-CeO2 catalyst derived from MOF template. Chem. Eng. J. 2019, 371, 78–87. [Google Scholar] [CrossRef]
  73. Sudarsanam, P.; Hillary, B.; Amin, M.H.; Abd Hamid, S.B.; Bhargava, S.K. Structure-activity relationships of nanoscale MnOx/CeO2 heterostructured catalysts for selective oxidation of amines under eco-friendly conditions. Appl. Catal. B 2016, 185, 213–224. [Google Scholar] [CrossRef]
  74. Zhou, C.; Zhang, H.; Zhang, Z.; Li, L. Improved reactivity for toluene oxidation on MnOx/CeO2-ZrO2 catalyst by the synthesis of cubic-tetragonal interfaces. Appl. Surf. Sci. 2021, 539, 148188. [Google Scholar] [CrossRef]
  75. Zhu, K.; Yan, W.; Liu, S.; Wu, X.; Cui, S.; Shen, X. One-step hydrothermal synthesis of MnOx-CeO2/reduced graphene oxide composite aerogels for low temperature selective catalytic reduction of NOx. Appl. Surf. Sci. 2020, 508, 145024. [Google Scholar] [CrossRef]
  76. Chang, H.; Li, J.; Chen, X.; Ma, L.; Yang, S.; Schwank, J.W.; Hao, J. Effect of Sn on MnOx–CeO2 catalyst for SCR of NOx by ammonia: Enhancement of activity and remarkable resistance to SO2. Catal. Commun. 2012, 27, 54–57. [Google Scholar] [CrossRef]
  77. Lin, F.; Wu, X.; Liu, S.; Weng, D.; Huang, Y. Preparation of MnOx–CeO2–Al2O3 mixed oxides for NOx-assisted soot oxidation: Activity, structure and thermal stability. Chem. Eng. J. 2013, 226, 105–112. [Google Scholar] [CrossRef]
  78. Zhang, H.; Gu, F.; Liu, Q.; Gao, J.; Jia, L.; Zhu, T.; Chen, Y.; Zhong, Z.; Su, F. MnOx–CeO2 supported on a three-dimensional and networked SBA-15 monolith for NOx-assisted soot combustion. RSC Adv. 2014, 4, 14879–14889. [Google Scholar] [CrossRef]
  79. Guo, L.; Meng, F.; Zeng, Y.; Jia, Y.; Qian, F.; Zhang, S.; Zhong, Q. Catalytic ozonation of NOx into HNO3 with low concentration ozone over MnOx-CeO2/TiO2: Two-phase synergistic effect of TiO2. Mol. Catal. 2020, 493, 111095. [Google Scholar] [CrossRef]
  80. Liu, Q.; Fu, Z.; Ma, L.; Niu, H.; Liu, C.; Li, J.; Zhang, Z. MnOx-CeO2 supported on Cu-SSZ-13: A novel SCR catalyst in a wide temperature range. Appl. Catal. A Gen. 2017, 547, 146–154. [Google Scholar] [CrossRef]
  81. Ma, D.; Yang, L.; Huang, B.; Wang, L.; Wang, X.; Sheng, Z.; Dong, F. MnOx–CeO2@TiO2 core-shell composites for low temperature SCR of NOx. N. J. Chem. 2019, 43, 15161–15168. [Google Scholar] [CrossRef]
  82. Zhang, L.; Cui, S.; Guo, H.; Ma, X.; Luo, X. The influence of K+ cation on the MnOx-CeO2/TiO2 catalysts for selective catalytic reduction of NOx with NH3 at low temperature. J. Mol. Catal. A-Chem. 2014, 390, 14–21. [Google Scholar] [CrossRef]
  83. Wu, X.; Yu, X.; Huang, Z.; Shen, H.; Jing, G. MnOx-decorated VOx/CeO2 catalysts with preferentially exposed {110} facets for selective catalytic reduction of NOx by NH3. Appl. Catal. B 2020, 268, 118419. [Google Scholar] [CrossRef]
  84. Lee, S.M.; Park, K.H.; Hong, S.C. MnOx/CeO2–TiO2 mixed oxide catalysts for the selective catalytic reduction of NO with NH3 at low temperature. Chem. Eng. J. 2012, 195, 323–331. [Google Scholar] [CrossRef]
  85. Sun, L.; Shao, R.; Chen, Z.; Tang, L.; Dai, Y.; Ding, J. Alkali-dependent synthesis of flower-like ZnO structures with enhanced photocatalytic activity via a facile hydrothermal method. Appl. Surf. Sci. 2012, 258, 5455–5461. [Google Scholar] [CrossRef]
  86. Dong, W.; Wang, X.; Li, B.; Wang, L.; Chen, B.; Li, C.; Li, X.; Zhang, T.; Shi, Z. Hydrothermal synthesis and structure evolution of hierarchical cobalt sulfide nanostructures. Dalton Trans. 2011, 40, 243–248. [Google Scholar] [CrossRef]
  87. Zhang, P.; Yin, S.; Sato, T. Synthesis of high-activity TiO2 photocatalyst via environmentally friendly and novel microwave assisted hydrothermal process. Appl. Catal. B 2009, 89, 118–122. [Google Scholar] [CrossRef]
  88. Wang, X.; Xu, H.; Wang, H.; Yan, H. Morphology-controlled BaWO4 powders via a template-free precipitation technique. J. Cryst. Growth 2005, 284, 254–261. [Google Scholar] [CrossRef]
  89. Yuan, Z.; Li, R.; Meng, F.; Zhang, J.; Zuo, K.; Han, E. Approaches to enhancing gas sensing properties: A review. Sensors 2019, 19, 1495. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  90. Bhargava, R.; Khan, S. Effect of reduced graphene oxide (rGO) on structural, optical, and dielectric properties of Mg(OH)2/rGO nanocomposites. Adv. Powder Technol. 2017, 28, 2812–2819. [Google Scholar] [CrossRef]
  91. Zhang, Y.; Huo, Q.Y.; Du, P.P.; Wang, L.Z.; Zhang, A.Q.; Song, Y.H.; Lv, Y.; Li, G.Y. Advances in new cathode material LiFePO4 for lithium-ion batteries. Synth. Met. 2012, 162, 1315–1326. [Google Scholar] [CrossRef]
  92. Amri, A.; Jiang, Z.T.; Pryor, T.; Yin, C.Y.; Djordjevic, S. Developments in the synthesis of flat plate solar selective absorber materials via sol–gel methods: A review. Renew. Sust. Energ. Rev. 2014, 36, 316–328. [Google Scholar] [CrossRef] [Green Version]
  93. He, F.; Wang, X.; Wu, D. New approach for sol-gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor. Energy 2014, 67, 223–233. [Google Scholar] [CrossRef]
  94. Zegeye, T.A.; Tsai, M.C.; Cheng, J.H.; Lin, M.H.; Chen, H.M.; Rick, J.; Su, W.N.; Jeffrey Kuo, C.F.; Hwang, B.J. Controllable embedding of sulfur in high surface area nitrogen doped three dimensional reduced graphene oxide by solution drop impregnation method for high performance lithium-sulfur batteries. J. Power Sources 2017, 353, 298–311. [Google Scholar] [CrossRef]
  95. Chen, X.; Gao, H.; Yang, M.; Dong, W.; Huang, X.; Li, A.; Dong, C.; Wang, G. Highly graphitized 3D network carbon for shape-stabilized composite PCMs with superior thermal energy harvesting. Nano Energy 2018, 49, 86–94. [Google Scholar] [CrossRef]
  96. Huang, X.; Shen, T.; Zhang, T.; Qiu, H.; Gu, X.; Ali, Z.; Hou, Y. Efficient oxygen reduction catalysts of porous carbon nanostructures decorated with transition metal species. Adv. Energy Mater. 2020, 10, 1900375. [Google Scholar] [CrossRef]
  97. Wu, X.; Liu, S.; Lin, F.; Weng, D. Nitrate storage behavior of Ba/MnOx–CeO2 catalyst and its activity for soot oxidation with heat transfer limitations. J. Hazard. Mater. 2010, 181, 722–728. [Google Scholar] [CrossRef] [PubMed]
  98. Ding, L.; Liu, C.; Chen, K.; Huang, Y.; Diao, B. Atmospheric pollution reduction effect and regional predicament: An empirical analysis based on the Chinese provincial NOx emissions. J. Environ. Manag. 2017, 196, 178–187. [Google Scholar] [CrossRef]
  99. Obradović, B.M.; Sretenović, G.B.; Kuraica, M.M. A dual-use of DBD plasma for simultaneous NOx and SO2 removal from coal-combustion flue gas. J. Hazard. Mater. 2011, 185, 1280–1286. [Google Scholar] [CrossRef]
  100. Song, C.L.; Bin, F.; Tao, Z.M.; Li, F.C.; Huang, Q.F. Simultaneous removals of NOx, HC and PM from diesel exhaust emissions by dielectric barrier discharges. J. Hazard. Mater. 2009, 166, 523–530. [Google Scholar] [CrossRef]
  101. Park, S.J.; Kim. B.J. A study on NO removal of activated carbon fibers with deposited silver nanoparticles. J. Colloid Interface Sci. 2005, 282, 124–127. [Google Scholar] [CrossRef] [PubMed]
  102. Park, S.J.; Kim, J.J.; Kim, M.J.; Park, R.J.; Cheong, H.B. Characteristics of flow and reactive pollutant dispersion in urban street canyons Cheong. Atmospheric Environ. 2015, 108, 20–31. [Google Scholar] [CrossRef]
  103. Park, B.J.; Park, S.J.; Ryu, S.K.J. Removal of NO over copper supported on activated carbon prepared by electroless plating. J. Colloid Interface Sci. 1999, 217, 142–145. [Google Scholar] [CrossRef] [PubMed]
  104. Park, S.J.; Kim, B.J. Oxidation behaviors of metallic copper particles in NO reduction mechanism of copper/activated carbons. J.Colloid Interface Sci. 2005, 292, 493–497. [Google Scholar] [CrossRef] [PubMed]
  105. Wu, X.; Liu, S.; Weng, D.; Lin, F.; Rin, R. MnOx–CeO2–Al2O3 mixed oxides for soot oxidation: Activity and thermal stability. J. Hazard. Mater. 2011, 187, 283–290. [Google Scholar] [CrossRef] [PubMed]
Catalysts 11 01519 g001 550
Figure 1. Mechanism for the SCR process of MnOx/CeO2. Reproduced with permission from Reference [45]; copyright (2017), Elsevier.
Figure 1. Mechanism for the SCR process of MnOx/CeO2. Reproduced with permission from Reference [45]; copyright (2017), Elsevier.
Catalysts 11 01519 g001
Catalysts 11 01519 g002 550
Figure 2. Preparation procedure of the carrier catalysts. Reproduced with permission from Reference [46]; copyright (2012), Elsevier.
Figure 2. Preparation procedure of the carrier catalysts. Reproduced with permission from Reference [46]; copyright (2012), Elsevier.
Catalysts 11 01519 g002
Catalysts 11 01519 g003 550
Figure 3. Schematic representation of the synthesis processes of mesoporous VOx-MnOx/CeO2. Reproduced with permission from Reference [30]; copyright (2020), Elsevier.
Figure 3. Schematic representation of the synthesis processes of mesoporous VOx-MnOx/CeO2. Reproduced with permission from Reference [30]; copyright (2020), Elsevier.
Catalysts 11 01519 g003
Catalysts 11 01519 g004 550
Figure 4. SEM images of Mn-CeO2 (HT) (a,b) and Mn-CeO2 (CP) (c), and TEM image (d,e) and high resolution TEM image (f) of Mn-CeO2 (HT). Reproduced with permission from Reference [26]; copyright (2016), Elsevier.
Figure 4. SEM images of Mn-CeO2 (HT) (a,b) and Mn-CeO2 (CP) (c), and TEM image (d,e) and high resolution TEM image (f) of Mn-CeO2 (HT). Reproduced with permission from Reference [26]; copyright (2016), Elsevier.
Catalysts 11 01519 g004
Catalysts 11 01519 g005 550
Figure 5. The schematic diagram of the catalytic mechanism of MnOx-CeO2/RGA Reproduced with permission from Reference [75]; copyright (2019), Elsevier.
Figure 5. The schematic diagram of the catalytic mechanism of MnOx-CeO2/RGA Reproduced with permission from Reference [75]; copyright (2019), Elsevier.
Catalysts 11 01519 g005
Catalysts 11 01519 g006 550
Figure 6. K+ poisoning process of the catalyst. Reproduced with permission from Reference [82]; copyright (2014), Elsevier.
Figure 6. K+ poisoning process of the catalyst. Reproduced with permission from Reference [82]; copyright (2014), Elsevier.
Catalysts 11 01519 g006
Catalysts 11 01519 g007 550
Figure 7. The possible mechanism of the VOx-MnOx/CeO2 catalyst. Reproduced with permission from Reference [83]; copyright (2020), Elsevier.
Figure 7. The possible mechanism of the VOx-MnOx/CeO2 catalyst. Reproduced with permission from Reference [83]; copyright (2020), Elsevier.
Catalysts 11 01519 g007
Catalysts 11 01519 g008 550
Figure 8. (a) TEM images, (b) HRTEM images, (c–h) energy spectrum analysis (EDS) images and corresponding element mapping of VOx-MnOx/CeO2. Reproduced with permission from Reference [30]; copyright (2020), Elsevier.
Figure 8. (a) TEM images, (b) HRTEM images, (c–h) energy spectrum analysis (EDS) images and corresponding element mapping of VOx-MnOx/CeO2. Reproduced with permission from Reference [30]; copyright (2020), Elsevier.
Catalysts 11 01519 g008
Catalysts 11 01519 g009 550
Figure 9. The possible mechanism of the VOx-MnOx/CeO2 catalyst. Reproduced with permission from Reference [30]; copyright (2020), Elsevier.
Figure 9. The possible mechanism of the VOx-MnOx/CeO2 catalyst. Reproduced with permission from Reference [30]; copyright (2020), Elsevier.
Catalysts 11 01519 g009
Table
Table 1. The preparation of MnOx/CeO2 binary catalysts.
Table 1. The preparation of MnOx/CeO2 binary catalysts.
CatalystConstructing StrategyPrecursorsCatalytic ActivityReference
MnOx/CeO2co-precipitation methodMn(NO3)2 Ce(NO3)3·6H2ONO and toluene conversion > 60% temperature < 300 °C[61]
oxidation-reduction reactionCe(NO3)3·6H2O KMnO4MnOx-CeO2-B achieve
96% NOx conversion at 100 °C at a space velocity of 30000 h−1		[35]
co-precipitation methodMn(NO3)2 Ce(NO3)3MnOx(0.4)-CeO2(500) achieve
95% NOx conversion at 150 °C		[27]
hydrothermal methodCeCl3·7H2O Mn(NO3)2MnOx-CeO2 microsphere catalyst presented
higher catalytic activity at low temperatures than
catalyst without shell-in-shell microsphere structure		[26]
chemical deposition methodMn(CH3COO)2 Ce(NO3)3CeO2/MnOx = 0.6 exhibited a relatively high conversion of NO[64]
hydrothermal methodCe(NO3)3·6H2O Mn(CH3COO)2·4H2OThe NO conversion of MCN achieves a conversion of about almost 100% in the whole 125–250 °C range[63]
co-precipitation methodMn(ac)2·4H2O Ce(ac)3·0.5H2OThe NOx conversion of 6Mn4Ce-C reached its maximum of 99% at 170 °C; and it’s NO conversion rate remained >80% at the temperature range of 120–275 °C.[62]
Table
Table 2. Summary of MnOx/CeO2-based ternary catalysts for NOx removal.
Table 2. Summary of MnOx/CeO2-based ternary catalysts for NOx removal.
CatalystPreparation MethodTesting ConditionCatalytic PerformanceReference
MnOx/CeO2/grapheneHydrothermal methodA fixed bed continuous flow reactor at 40–160 °C, the GHSV of 24,000 h−1.NOx conversion on MnOx-CeO2(8:1)/GR catalyst achieve 99.3% at 80 °C
N2 selectivity nearly 95% at 80 °C		[25]
VOx/MnOx/CeO2Impregnation methodsCatalysts (200 mg, 40–60 mesh), a GHSV of 160,000 h −1, a flow rate of 600 mL/minNH3-SCR activity (NO conversion > 95%)[30]
MnOx/CeO2/MOFHydrothermal method50 mg catalyst, the flow rate of 50 mL/min, GHSV of 60,000 mL/gcath.Toluene conversion was 86%[34]
MnOx/CeO2/ZrO2Precipitation methodThe flow rate of 420 mL/min, the GHSV by volume was10,000 h−1.NOx conversion is over 80%[46]
MnOx/CeO2/reduced graphene oxideHydrothermal methodThe flow rate was 500 mL·min−1, a GHSV of 30,000 h−1.NOx conversion (~99%) could be attained at 220 °C[75]
SnO2/MnOx/CeO2Precipitation method0.20g catalyst, a gas flow rate of 100 mL min−1, GHSV of 3.5 × 104 h−1More than 98% NO conversion was obtained at 80 °C and nearly 100% NO conversion at the temperature range of 110–230 °C[76]
MnOx/CeO2/Al2O3Sol–gel method100 mg of catalyst, the reactor temperature was ramped to 650 °C.The NO oxidation activities achieves 99%[77]
MnOx/CeO2/SBA-15Sol–gel methodCatalyst (200 mg, 10–20 mesh), reaction was run from 100 to 650 °Csoot oxidation with 100% selectivity to CO2[78]
Ba/MnOx/CeO2Impregnation method10 mg of catalysts, test was at 600 °C (heating rate 15 °C/min)The amount of NOx desorbed from BaMnCe within the temperature interval of 350–450 °C is 2.8 times of that MnCe[79]
MnOx/CeO2/TiO2Impregnation method0.20 g catalyst (20–40 mesh), a GHSV of 30,000 h −1the NOx removal efficiency achieves up to 77.1%[79]
MnOx/CeO2/Cu-SSZ-13impregnation methodN2 was the balanced gas, and total flow rate = 300 mL min −1The NOx conversions were above 90% from 125 °C to 450 °C[80]
MnOx/CeO2@TiO2hydrothermal methodThe air flow rate was 1600 mL/min and the airspeed was 24,000 h−1The NOx conversion of MnOx/CeO2@TiO2 reaches around 100% at 140 °C. The N2 selectivity of MnOx/CeO2@TiO2 remains above 95% from 40 to 200 °C.[81]
MnOx/CeO2/TiO2Co-precipitation methodThe GHSV was 27 000 h −1, and the catalytic temperature was at the range of 80–300°C.The NOx conversions reaches 87% at 150 °C and the NO conversion is 85% at the temperature of 300 °C[82]
MnOx/CeO2/VOximpregnation methodThe catalyst (200 mg, 40–60 mesh), the GHSV was 160,000 h −1VOx-MnOx/CeO2-R achieve > 95 % NO conversion at ∼220 °C.[83]
MnOx/CeO2/TiO2impregnation methodThe total flow was 500 cc/min, and GHSV of 60,000 h−1Mn(20)/Ce(4)–TiO2 catalyst obtained 90% NOx conversion at 180 °C and >94% N2 selectivity[84]
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Sun, H.; Park, S.-J. Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review. Catalysts 2021, 11, 1519. https://doi.org/10.3390/catal11121519

AMA Style

Sun H, Park S-J. Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review. Catalysts. 2021; 11(12):1519. https://doi.org/10.3390/catal11121519

Chicago/Turabian Style

Sun, Hao, and Soo-Jin Park. 2021. "Recent Advances in MnOx/CeO2-Based Ternary Composites for Selective Catalytic Reduction of NOx by NH3: A Review" Catalysts 11, no. 12: 1519. https://doi.org/10.3390/catal11121519

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop